1. Technical Field
The present disclosure generally relates to filtering and conditioning components for power systems.
2. Description of the Related Art
Direct current-to-direct current (DC/DC) converters that operate from vehicle power (e.g., spacecraft, aircraft, missiles, ground vehicles, marine applications) are designed to operate over a steady state voltage range around a nominal operating point, e.g., 28 V dc, and to accommodate transients at their input. The transients can be characterized as surges and sags in input voltage and voltage spikes of one sort or another. Generally, surges and sags are voltage increases/decreases that are of relatively long duration from sources with relatively low source impedance. Spikes are of much shorter duration, have higher amplitude, may be of either polarity, and are usually limited in energy.
Switch-mode power converters (SMPCs) are a source of electromagnetic interference (EMI) in electronic equipment. In order to comply with strict electromagnetic compatibility (EMC) requirements, an EMI filter is often needed at the input of a SMPC. The design of EMI filters aims at achieving required insertion loss (IL), i.e. attenuation of the power of the unwanted electromagnetic emissions (EME) from a switch-mode DC/DC converter.
SMPCs are the most widely used DC power supplies because SMPCs are significantly smaller, lighter and more efficient than linear power supplies. The main drawback of SMPCs is the current related electromagnetic interference (EMI) at their inputs and voltage related interference at their outputs. The requirements of the load dictate the design of the output filter, which is an important part of the design of the converter and its controller. The input EMI filter, on the other hand, is normally not necessary for the operation of the converter itself. The design of the EMI filter aims at achieving required insertion loss (IL), i.e. attenuation of the power of the unwanted electromagnetic emissions (EME) from a switch-mode DC/DC converter. The task of the input filter is to ensure EMC within the system or with neighboring systems, and to comply with relevant EMC standards.
FIG. 1 shows an example of an EMI filter 10 positioned between a switching power converter 12 and an input voltage source 14. Assuming the components of the EMI filter 10 do not vary with time, the EMI filter may be considered a passive linear electrical circuit.
The EMI filter 10 includes a single common mode filter stage 16 that includes a common mode EMI choke or inductor LCM and two bypass capacitors CY1 and CY2. The EMI filter 10 also includes a single differential mode filter stage 18 that includes a differential mode EMI inductor LDM1 and a bypass capacitor CX1. The EMI filter 10 is coupled between input terminals 20, 22 of the switching power converter 12 (or other noise source) and output terminals 24, 26 of the input power source 14. The power converter 112 generates common mode noise and differential mode noise.
When a differential current such as the normal operation current of the switching power converter 12 passes through the common mode EMI inductor LCM, the differential current cancels out in two windings of the common mode EMI inductor LCM. As a result, there is no net magnetization of the core of the common mode EMI inductor LCM. Consequently, the common mode EMI inductor LCM has no impact on the differential current. In contrast, when a common mode noise current passes through the common mode EMI inductor LCM, the common mode noise current magnetizes the core of the common mode EMI inductor LCM. As a result, the common mode EMI inductor LCM show high impedance for the common mode noise current so as to prevent the common mode noise current from polluting the input power source.
Two common mode bypass capacitors CY1 and CY2 are connected in series and coupled between the two input terminals of the power converter 112. A joint node 28 of the two common mode bypass capacitors CY1 and CY2 is coupled to ground. The common mode bypass capacitors CY1 and CY2 conduct common mode noise generated by the power converter 12 to ground.
The differential mode EMI choke or inductor LDM1 is coupled between the common mode EMI inductor LCM and the positive input 20 of the power converter 12. The differential mode EMI inductor LDM1 suppresses differential mode noise generated by the power converter 12.
The differential mode bypass capacitor CX1 is coupled between the input terminals 20 and 22 of the power converter 12 and the differential mode EMI inductor LDM1i and is connected in parallel with the common mode bypass capacitors CY1 and CY2.
Generally, EMI filtering is required between a switching power converter and an input voltage source to achieve compliance to regulatory limits on conducted emissions. For example, the MIL-STD-461 defines the EMI requirements for subsystems and equipment used in military applications. European Community Directive on EMC, Euro-Norm EN 55022 or 55081 is another common industry standard on conducted radio frequency emissions.
The power converters typically used in commercial aerospace and military applications may be designed for applications where the input voltage source is 28 V DC input (nominal). The input voltage source is normally defined by a specification or standard. For military airborne applications, this standard may be MIL-STD-704. For military ground applications, the standard may be MIL-STD-1275, and for shipboard applications the standard may be MIL-STD-1399. Commercial aerospace applications may use the MIL-STD-704 standard and more often the DO-160 standard. Some commercial aerospace applications may utilize standards where the 28 V DC source was defined by the GJB181-1986 and GJB181A-20 standards.
Each of these source requirement documents defines a steady-state range of input voltage and a range of transient voltages that the load equipment (e.g., power converter) is required to accept. Each of these documents has a number of revisions that over time typically evolve toward tighter limits on the transient extremes and sometimes on the steady-state range as well as generation equipment, and the controls for them, have improved. However, when a product is being developed, it is desired that the product be capable of operating over the widest range possible so that the product can be used on the largest number of platforms, including those developed when early versions of the source specification was in place. This is also typical for custom products since many platforms remain in service for many years, and particularly when the application is targeted for sale to multiple platforms.
For example, the steady-state range for 28 V input sources across these documents is typically from a narrowest range of 22 V DC to 29 V DC and a widest range of 18 V DC to 36 V DC. The widest transient voltage range is from MIL-STD-1275, which specifies a high transient level of 100 V and a low transient level of 6 V (when operation during cranking is required).
Many standard DC/DC converters are designed to operate from 28 V input sources accept 15-40 V steady-state and 50 V transient. Some DC/DC converters may be designed for 15-50 V steady-state and 80 V transient, for example.
In order to use such power converters with input sources that have wider steady-state and transient requirements, three additional functions may need to be added to the converters, namely, an EMI filter, a boost converter (e.g., for low transients between 8V and 15V), and a transient limiter (e.g., for high transients between 50 V and 80 V). These functions may be added to the power converters as separate functional components.
The wide input range required for power converters is generally accomplished either by designing the power converter to operate over the full input range or by adding separate boost and or transient clipping/limiting functional blocks, depending on the required input range and the designed input range of the power conversion stage. Requiring a power converter to operate over the full input range when the input power source has a very wide input range (e.g., as much as 10:1) results in compromises to the power converter design, including lower efficiency. Such designs also prevent use of existing available converters that are not designed for the required wide input range. Moreover, use of separate functional blocks for EMI compliance, power conversion, and low-side (boost) or high side (clipping/limiting) functions can achieve good electrical performance but generally requires greater volume and higher cost.